An exemplary fuel cell, as presented in FIG. 1, includes an anode side 100 and a cathode side 102 and an electrolyte material 104 between them. In solid oxide fuel cells (SOFCs) oxygen is fed to the cathode side 102 and it is reduced to a negative oxygen ion by receiving electrons from the anode. The negative oxygen ion goes through the electrolyte material 104 to the anode side 100 where it reacts with the used fuel producing water and, for example, carbon dioxide (CO2). Between the anode 100 and the cathode 102 is an external electric circuit 111 having a load 110 for the fuel cell.
FIG. 2 shows a SOFC device as an example of a high temperature fuel cell device. SOFC devices can utilize for example natural gas, bio gas, methanol or other hydrocarbon containing compounds as fuel, which is fed via feed for gas used as fuel 108 to the anode side(s) 100. The SOFC device system in FIG. 2 includes multiple fuel cells in one or more stack formations 103 (SOFC stack(s)). A larger SOFC device system includes many fuel cells in several stacks 103. Each fuel cell includes anode 100 and cathode 102 structures as presented in FIG. 1. Part of the used fuel may be recirculated in feedback arrangement 109. SOFC device in FIG. 2 also comprises a fuel heat exchanger 105 and a reformer 107. Heat exchangers are used for controlling thermal conditions in the fuel cell process and there can be more than one of them in different locations of a SOFC device. The extra thermal energy in circulating gas is recovered in one or more heat exchangers 105 to be utilized in the SOFC device or externally. Reformer 107 is a device that converts the fuel such as for example natural gas to a composition suitable for fuel cells, for example to a composition containing all or at least some of the following: hydrogen, methane, carbon dioxide, carbon monoxide, inert gases and water. Anyway in each SOFC device it is though not necessary to have a reformer.
By using a measurement means 115 (such as a fuel flow meter, current meter, temperature meter and the like) desired measurements for the operation of the SOFC device are carried out. Only part of the anode exhaust gas is recirculated in the feedback arrangement 109 and the other part of the gas 114 is oxidized in a post oxidation device such as a burner.
Fuel cells are electrochemical devices converting the chemical energy of reactants directly to electricity and heat. Fuel cell systems have the potential to significantly exceed the electrical and CHP (Combined production of Heat and Power) efficiency of known energy production technologies of comparable size. Fuel cell systems are widely appreciated as a key future energy production technology.
In the solid oxide fuel cell (SOFC) system, for example a partially reformed hydrogen rich fuel gas mixture is fed to the anode side of the fuel cells while air is lead to the cathode sides. Fuel oxidation reactions take place and hydrogen and other oxidizable compounds are converted into water and carbon dioxide while electric current is generated. Since reforming of hydrocarbon fuel involves steam, it is beneficial to recover water formed as the product of the fuel oxidation and to use the water for fuel reforming in the reformer 107, thus omitting a need for an external water feed to the system once the system is already up and generating electricity.
A practical method for recovering water formed as the product of fuel oxidation reactions in the fuel cell is anode off-gas recirculation. This method can improve overall fuel utilization compared to single passing operation of the fuel cells.
In known anodes, off-gas recirculation involves a compressor or other device for creating a pressure boost enough to overcome pressure drops in the fuel cell system and to provide mass flow of water vapour adequate for fuel steam reforming, a key control parameter being an Oxygen-to-Carbon (O/C) ratio of the fuel gas mixture.
In one known system embodiment, a high pressurized fuel feed is used as a motive stream in an jet-ejector to entrain anode tail gas and to increase pressure of the fuel gas mixture to overcome pressure losses in the fuel cell system components. For example in patent application document JP2008282599 (A) is presented this kind of system topology. These kinds of system topologies involve high pressure of the fuel feedstock and due to the fixed geometry of the jet-ejector, these topologies have a limited capability for controlling the re-circulation ratio and the resultant Oxygen-to-Carbon (O/C) ratio.
Recirculation carried out by a fan or a compressor provides added flexibility and controllability to the system but involves sophisticated, complex and potentially unreliable machinery. Both of the aforementioned methods often rely on inferred and thus inaccurate determination of Oxygen-to-Carbon (O/C) ratio since measurement of high temperature gas stream composition can be difficult and complicated.